The present invention pertains to an optical isolator intended to be integrated into optical devices, such as optoelectronic components, and, in particular, photonic integrated circuits, or PICs. These optical devices are used mainly in the field of high-speed digital telecommunications. The invention also extends to the method for manufacturing such an optical isolator.
An optical isolator is a nonreciprocal passive device that has high attenuation in one direction and low attenuation in the opposite direction; it makes it possible to transmit light in only one direction, but prevents light from propagating in an opposite direction. For this reason, optical isolators are essential elements for eliminating the negative effects of reflected and parasitic light beams in fiber-optic transmission, particularly at high speeds.
Today, commercial products do not contain any optical isolators integrated into their chips. Manufacturers of optoelectronic components, particularly photonic integrated circuits, or PICs, await a solution. In the remainder of the text, the term “integrated component” refers to a component that is monolithically incorporated into a device, meaning a component supported by a substrate that is shared with the device's components.
Two types of optical isolators have been studied. A first type of optical isolator implements a nonreciprocal optical effect so that it can serve as an optical isolator, the most well-known of which is the Faraday effect. When subjected to an outside magnetic field, some materials, known as magneto-optical materials, change the light's polarization direction. An optical isolator of this first type is normally composed of (i) a magnetic garnet crystal that has a Faraday effect, (ii) a permanent magnet for applying a defined magnetic field, and (iii) polarizing elements that allow only light that has a given direction of polarization (incident light) to pass through, while blocking the passage of light that has a direction of polarization orthogonal to that of the incident light. This first type of optical isolator is hard to integrate into optical devices, as it requires multiple components whose integrated version is not yet available.
There is a second type of optical isolator, an absorption optical isolator, whose complex optical index is nonreciprocal. In the presence of a magnetic field, the optical index of some ferromagnetic materials, such as an iron-cobalt metal alloy, depends on the direction in which the light propagates. Depending on its direction of propagation, the light will consequently be more attenuated or less. This second type of optical isolator is well suited to being integrated into optical devices that particularly comprise a semiconductor laser source.
In this second sort of optical isolator, the ferromagnetic metals introduce high optical power losses, often greater than 20 dB, including in the incident direction. Semiconductor optical amplifiers, or SOAs, must therefore also be integrated in order to compensate for these optical power losses. Thus, the amplification of the optical signal provided by the SOA makes it possible to offset the ferromagnetic material's optical power losses in a first direction of incident light propagation. In the direction opposite the propagation, the ferromagnetic material's attenuation remains predominant, in order to keep the light from traveling backwards.
And optical isolator of this second type normally comprises an optical amplifier SOA made up of a stack (i) of an n-doped semiconductor material layer, (ii) an amplifying active part, which has an optical index greater than the layers that surround it, and (iii) a p-doped semiconductor material layer. In order to integrate the optical isolator, the ferromagnetic metals may be deposited onto the top or sides of the SOA's waveguide.
In the event that ferromagnetic metals are deposited onto the top of the SOA's waveguide, the strip's height is very critical, as the optical signal's propagation mode needs to interact with the ferromagnetic metals. This is because the waveguide's thickness determines the distance separating the ferromagnetic metals from the amplifying active layer. In this case, there will be considerable optical power losses due both to the electrical contact layers, often made of an InGaAs, ternary semiconducting material, and to the electrical contact-conveying metallic layer, which is placed above the waveguide.
If the ferromagnetic metals are deposited on the sides of a SOA's waveguide, the ferromagnetic metal layer depositing operation, generally carried out in steps of depositing following by steps of etching, is the very critical operation. The step of removing material by etching turns out to be very difficult to carry out in this type of optical isolator.